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Author Topic: Using a torque meter for a twin  (Read 913 times)
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tom arnold
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« on: August 24, 2016, 03:27:20 PM »

I have been using a torque meter for the last year or so in flying my models and have been absolutely delighted with the results. This has all been with single engined models and I started after flying with Don DeLoach for a number of years and watching his overall success with flying scale models in general. At a Geneseo Nats a couple of years ago I was his mechanic as he won 5, yes 5, mass launches in a row. I was astounded and after being hit over the head with this 2X4 I figured I could learn something from his methods. Like all the guys in the FAC he was more than willing to share his knowledge and techniques and among them is the use of a torque meter which he learned from flying competition free flight. He subsequently published 2 articles in the Flying Aces Newsletters entitled "One Flyer's Approach to Better Performace", Part I and Part II and found in the Sept-Oct 2012 and the Nov-Dec 2012 issues. These articles, if you want to really up your performance and enjoyment, should be absolutely memorized by a modeler. I have read them again and again to get the concepts in my head. In the articles he makes the case why torque measurement is so much more USEFUL than counting turns. I have since taken my expensive turn counter off my winder as it really does me no good after using a torque meter.

In a nutshell, a torque meter tells you what power is going to come out of the nose accurately and reliably every time. It does not matter what kind of power you have in your model whether it is neatly braided 1/8" strip, an inner tube, a broken motor, or squirrel running in a cage. The winding is done to pack all the available winds that could possibly be put in the motor and what that number is, is meaningless as you can't get any more in anyway. Guys will say "I am flying on 1200 winds" which is smoke in the air as it is 1200 winds of what? 2 loops of 1/8"? 10 loops of 1/4" And how long? 8", 15" 36"? all have differing effects on the released power. The number of prior flights has an effect as with each winding to 1200, the released power goes down as the motor get tired. I will let you read Don's articles. I subsequently had an article published in the FAC newsletter too on how to trim a model using a cook-book approach and a torque meter titled "Torque, Trimming, Voo Doo and Beethoven". I can't remember the issue but if any one wants it, email me and I'll send it.

So, on to twins. I at first was leery of it as reading a torque meter is a moving needle and at first glance, the discrimination on the face of the meter doesn't look fine enough for get a consistent reading. In practice it is not an issue. As the torque goes up the needle does not wiggle but is locked hard at whatever torque is under strain at the moment. You only need to read the units to halves anyway, i.e. 4.0 inch-oz, 4.5 inch-oz, 5.0 inch-oz and so on. Both motors of a twin can be wound to practically identical torques reliably as a result. Even if you have mixed motors or a fresh one in one nacelle and an old one in the other, you can launch knowing that the pull power in both are matched even after many flights. You just cannot get that by counting winds as continual flights will weaken motors unevenly and you will have turn problems that defy solving......or at least I did.
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« Last Edit: August 24, 2016, 03:43:45 PM by tom arnold » Logged
Bredehoft
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« Reply #1 on: August 24, 2016, 04:05:20 PM »

Well put, Tom.  I never flew by torque until recently and I agree.  I still do a lot of less-than-serious flying without torque meters, but have adopted their use when I fly Stick, Fuselage, 2 Bit, Jimmie Allen, and Embryo.  I will start doing the same or like-sized scale planes and will eventually get to this in Peanut, too.  Consider me converted by Don DeLoach, too!

Good comments on twins.  The only caution I would use is if you are using right and left props - you need to find a torque meter that will read in both directions.  I haven't seen one yet (except a small one I made myself) that had a scale that would read in both directions - and many have a hard stop preventing the needle going in the opposite direction.

--george
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Don McLellan
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« Reply #2 on: August 24, 2016, 06:09:59 PM »

Hi George,

Can you post a pic of the bi-directional torque meter?

Don
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« Reply #3 on: August 24, 2016, 08:02:58 PM »

here you go!

http://volareproducts.com/wp-content/uploads/TorqueMeters.jpg

--george
Using a torque meter for a twin
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Copbait73
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« Reply #4 on: September 05, 2016, 10:03:10 PM »

Reading these comments let me say I fully accept the statements made but at some point the engineer in me kicks in and I begin to question the robustness of the actual process.
A big issue I have is this: OK you wind to a torque then what? How do you get the rubber off the winder, then off the blast tube wire and finally on to your typical S hook without letting out an unknown number of winds, therefore losing the torque which is desired?
If you say this is not critical, how do you know?
If your plane uses a crockett(sp?) hook or gizmoGeezer this is not a issue but many do not, especially the smallest ones.
Interested in hearing your feedback to this consideration.
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Hepcat
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« Reply #5 on: September 06, 2016, 08:25:24 AM »

Don,
If you are interested in making one yourself the following may help if your eyesight is good!  I cannot forget these two photographs unfortunately because they are so bad.  Some setting must have been completely off.  I have added a mostly completed CAD drawing which should help.  The meter is what most people build for themselves, a wire fixed at one end and carrying a dial pointer at the other.  On this meter I added a release button, just below the dial, which, if pushed in will allow all or some of the turns to be run off.  How this happens can be seen in the picture of the back end.  The torque wire is fixed to an 'L' shaped bit of wire which can swivel in a brass tube. However if the button is pulled out the wire from the button, which runs to the back of the meter, is shaped in such a way that the 'L' shaped bit is prevented from turning.  If the button is pushed in the wire shape allows the 'L' to turn and run off winds.  Now I have gone through all that I realize that almost any torque meter will work either way, you just need to put up and down scales on the dial!!!  Still I shall leave it all now I have done it.
John
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tom arnold
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« Reply #6 on: September 06, 2016, 10:54:54 AM »

Copbait has a good point. In transferring the motor to the prop there is, indeed, some torque lost. I try to minimize that with using a blast tube wire that is part of my torque meter (a BMJR unit--see his website for an example photo) so there is only one transfer. The other is that I use the same size O-ring on both motors and when making the transfer allow only the O-ring to unwind, not the rubber motor itself. This at least equalizes the torque loss on both sides. An interesting phenomena I noticed is that the drag of fully wound motor rubbing against the inside of a blast tube can give you a slightly higher torque reading. I compensate for that by sliding the blast tube on to the wire and then pausing to re-read my torque reading. It has usually dropped a bit with the "naked" motor so I crank on an extra turn or so to bring it up to my desired torque. I know, doing that is living dangerously as the blast tube is gone but I am winding below the bursting torque and I seldom use a motor more than 3-4 times. I am sure I am going to get caught someday!
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Don McLellan
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« Reply #7 on: September 06, 2016, 11:39:17 AM »

Hi John,

Thank you very much for the pictures and drawing of the torque meter.  I will take a stab at building one of these.

Don
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Hepcat
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« Reply #8 on: September 08, 2016, 07:01:37 PM »

Don, and Others thinking of using a Torque Meter on a twin.

Recently I have been flying the 'Legal Eagle' class Indoors.  Most Indoor models are wound with a torque meter and the then motor is transferred to the hooks on the motor stick and propeller.  However the 'LE' has a fuselage with the rubber inside so something different is needed.  What I have done is to convert an old torque meter  to check the torque on the propeller after the motor is wound in the normal way for a fuselage model.  The conversion is very simple.  If you look at the picture in reply#5 you will see the rubber hook just in front of the dial.  I took two pieces of balsa, 1/8 x 3/8 x 2.25" and stuck them together  with a 3/4" overlap on top of the hook.  I sanded a half round groove at the end of each strip, laid a cocktail stick in the grove and pulled a strip of paper over the stick and glued the paper to the balsa.  I can adjust the length of the sticks or take them out completely for travelling.  That is all there is to it.

In use one winds the rubber as normal, holds the propeller, puts the meter in place and then releases the propeller.  There is no need to be accurate in centering the meter. [It is an interesting mechanical engineering fact that torque on a bar measures the same anywhere along the bar.  To emphasize this I have posed the meter in one of the pictures in that way.

The way the balsa strips are glued together means that one side of each strip is on the centre line and can be used as a pointer.  However I am thinking of finding a piece of thin black wire and mounting it at 90 degrees to the wooden strips to form a better pointer.  
Please move to next reply as I have got mixed up on photograph sizes.  John
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« Reply #9 on: September 08, 2016, 08:05:07 PM »

Continuing from earlier reply on torque meters.

In reply #50 I gave a very precise figure on the CAD drawing but did not say why.  I calculate the length of wire required  so that the maximum torque I want to read with that meter occurs at one full dial turn of the pointer.  Now the twisting of the wire is so sensitive to changes in the wire diameter (even tenths of a thousandth matter) that the calculation is never correct so I initially make the wire about an inch too long with a short right angle bend near the end and clamp the bent end down firmly. Now I must remind you here that I am talking about a torque meter with a hook at the end to hold a rubber motor but at this juncture it becomes apparent that the two things are virtually identical; the plain torque meter for rubber has a hook on the end and the other has a bar fixed to the hook to engage the propeller.  But now the plain torque meter needs a temporary bar fitted on the hook that may be loaded with weights at the end to check the twisting of the wire.  The free end of the wire is cut back and clamped in a series of small steps until the desired full scale reading is reached.  The end is then soldered to its anchor.

I think that has over egged the pudding enough for now.
 
John
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David Lofthouse
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« Reply #10 on: January 05, 2017, 02:12:55 PM »

Reading these comments let me say I fully accept the statements made but at some point the engineer in me kicks in and I begin to question the robustness of the actual process.
A big issue I have is this: OK you wind to a torque then what? How do you get the rubber off the winder, then off the blast tube wire and finally on to your typical S hook without letting out an unknown number of winds, therefore losing the torque which is desired?
If you say this is not critical, how do you know?
If your plane uses a crockett(sp?) hook or gizmoGeezer this is not a issue but many do not, especially the smallest ones.
Interested in hearing your feedback to this consideration.


Sorry for coming in so late on this interesting topic.  I agree, if consistency is what we are trying to achieve, all we can hope for is "some" and never"total.  Tying your motor knot slightly longer, a thousandth of an inch difference in motor cross section, even the length of time one motor sits in a wound condition will have an effect.  But what came to mind when I read your remark was the rings we used when I was flying indoor.  Ray Harlan used to sell o rings for very small models, they had nicely rounded edges and were made of some sort of very tough nylon or the like.  I have also used rubber o rings but after a few failures I stopped.  The fly fishing/tying industry offers some very nice seamless titanium rings in small sizes.  To assure that you do not lose turns in the transfer process, many guys used a wire which was slipped through the ring giving you something to hang on to while moving from hook to hook.  I am happy with a crockett too, just throwing out some smaller lighter options.     
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tom arnold
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« Reply #11 on: January 13, 2017, 11:50:56 AM »

John, your indoor "Torque Checker" is really interesting and I am trying to figure out how I can use the concept for outdoor flying. However, I do have a question on torque meters in general. I have heard that to twist the wire beyond 270 degrees on the face permanently puts a "set" in the wire and it will no longer read the same. Is that true? It has been so long since my college Strength of Materials class, I have forgotten everything.
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